1. Technical Field
The present invention relates to a method of supplying high-voltage and large-capacity electric power from shore to a ship berthed in port, and a system thereof.
2. Description of the Related Prior Art
Generally, entire electric power for a ship is supplied by driving a generator with a turbine or engine as is known in e.g., Japanese Laid-Open Patent Publication 2000-217275. However, in a case that the ship is berthed in port for a long period of time or a short period of time, when the ship has equipment consuming a large amount of electric power such as a freezing container, an engine (generator) of the ship must be kept in operation while the ship is berthed. Consequently, a large amount of exhaust gas is generated, and it causes air pollution and marine pollution. In order to avoid such problems, it is recommended in recent years to stop the engine (generator) of the ship and to supply electric power from shore to the ship while the ship is berthed alongside the quay.
FIG. 2 illustrates a conventional electric power supply system in a ship. As illustrated in the figure, the ship is equipped with a main bus bar 62 to which generators 60, 61 are connected. Power lines 64 for large load such as cargo-handling equipment 63 (a freezing container, for example) are connected to the main bus bar 62 via circuit breakers 69. A power line 66 for propulsion auxiliary equipment 65 such as a bow thruster as disclosed in e.g., Japanese Laid-Open Patent Publication 2000-280985, a side thruster, etc. is connected to the main bus bar 62 via a circuit breaker 70. Also, power lines 68 for ordinary load 67 such as onboard lighting equipment, electronic devices, or the like (other load) are connected to the main bus bar 62 via circuit breakers 71. Circuit breakers 72, 73 disposed between the main bus bar 62 and the generators 60, 61, and the circuit breakers 69 to 71 are installed in a main switchboard 75. A reference number 74 denotes a step-up transformer disposed between the power line 66 and the propulsion auxiliary equipment 65.
Generally, when electric power supplied from the generators 60, 61 is changed over to the electric power from shore, supply of the electric power from the generators 60, 61 to the main bus bar 62 is halted, and then the electric power from shore is supplied to the main bus bar 62. Thus, a short-time interruption of power supply is inevitable.
However, in the above method of changing over power sources, the electric power supply to the ship is interrupted temporarily, and consequently there have been problems such that onboard control devices including computers are reset.
In order to avoid the problems, it may be devised to change over from a power source of generators to a power source supplied from shore after frequencies and voltages of the power sources are matched (i.e., synchronized). In case of a large ship, the electric power necessary for the ship is supplied by driving several generators in parallel, and complicated computer control has been taken in order to allocate load to the generators while keeping frequencies of the plural generators constant. In such case, adjusting the frequencies of the plural generators in operation to the frequency of the power source from shore has not been carried out generally, since such adjustment is very difficult. Furthermore, when a large capacity of electric power is supplied from shore, a power line for receiving the electric power from shore must be connected to a main switchboard, which not only requires large-scale modifications inside the ship but also imposes burdens on an entire system of the ship. In some cases, since the electric power from shore is too large, there is little space in the main switchboard to install necessary equipment.